In one type of optical scanner used for reading bar-coded labels, a scanning pattern is generated by using a rotating, multi-faceted hologram-bearing member to deflect a coherent light beam produced by a stationary laser. The deflected beam is typically redirected by one or more beam folding mirrors in a set of fixed mirrors to form the final pattern. The deflected beam leaves the deflecting element at different angles relative to the surface of the element so that it strikes different mirrors in the array or strikes the same mirrors at different points to produce noncoincident scan lines in the final pattern.
Scanners can operate in a retroreflective mode. That is, light reflected from a label will re-enter the scanner along the path of the outgoing beam. The reflected light will traverse at least part of that path before being collected at a photodetector. The photodetector converts the collected light into an electrical signal which is processed to find and decode a scanned label.
In known systems, the facets of the rotating, hologram bearing member are the same size. While this is advantageous from a manufacturing standpoint, uniform facets possess at least one disadvantage when used in a retroreflective scanner which generates a multidirectional scanning pattern. Different amounts of light are retroreflected from or through such facets depending upon the characteristics of the outgoing beam. The variations in collected light level produce corresponding variations in the electrical signal produced by the photodetector. The electronics used to process the electrical signal must be designed with a wide dynamic range to accommodate the variations in the electrical signal. This requirement adds to the cost and complexity of the system.